There are several known tensioner designs where the tensioner arm movement towards the drive belt is controlled with a one-way mechanism. U.S. Pat. No. 4,145,934 describes a wedge that is pushed against the arm eccentric (lever) so that latter cannot rotate outwards once the tensioner arm is biased towards the belt by a tensioning spring.
U.S. Pat. No. 4,351,636 describes a tensioner similar in principle except that the one-way wedge is replaced by a ratchet/pawl assembly.
Another ratchet and pawl mechanism is described in U.S. Pat. No. 4,634,407.
U.S. Pat. No. 4,392,280 describes a device where the one-way mechanism consists of either a one-way roller clutch or a spring clutch placed between the tensioner arm and the pivot shaft.
All of the above-mentioned inventions describe a one-way mechanism, which does not allow the tensioner arm rotate away from the belt once the arm is allowed to move inwards towards the belt.
U.S. Pat. No. 4,583,962 offers an improvement to these designs by describing a mechanism that allows a limited return stroke of the arm towards the backstop, required by the thermal expansion of the engine. The detail design of this patent describes a spring clutch type one-way device and an arc shape slot wherein the arm is free to rotate backwards.
U.S. Pat. No. 4,808,148 describes a design wherein the slot controlled reverse stroke is replaced by a resilient biasing element such as elastomeric spring located between the ratchet and pawl assembly and the stationary mounting member.
U.S. Pat. Nos. 4,822,322 and 4,834,694 describe inventions wherein the one-way mechanisms are conventional on-way (roller) clutches and the arm return strokes are controlled by arc shaped slots.
All of the above mentioned designs, even if they allow some limited return stroke to compensate thermal expansion of the engine block, do not allow a total return of the tensioner arm to pre-installation position, which will be required if there is a need to reinstall the current belt after a field service or even more so, if the installation of a new, unstretched belt is required.
There have been some proposals to overcome the reinstallation problem described above.
U.S. Pat. No. 4,917,655 has a plurality of arc shaped slots, which control the movement of the spring clutch and a suggestion that the spring clutch can be forced to move against its locked, non-return direction.
U.S. Pat. Nos. 4,923,435 and 6,375,588 describe tensioners wherein there is a viscous clutch installed in series with a one-way mechanism between the tensioner arm and stationery part of the tensioner. In these cases, the arm can be forced away from the belt by overcoming the viscous friction forces in the viscous clutches.
All of these designs have a serious drawback, since the return torques required to rotate the arm away from the belt are very high. In case of forcing the viscous clutch, if the turning speed is not very slow, the operator is most likely to damage some components in the tensioner. On the other hand, the reverse movement of a simple spring clutch, as described in U.S. Pat. No. 4,917,655, is most likely to result in bent spring tangs rather than release of the clutch. This is due to the fact that spring clutches are extremely sensitive to the manufacturing tolerances of the spring wire, the spring coils and the shaft diameter as well the level of the coefficient of friction which is highly unpredictable especially in dusty engine compartment environment in used, older cars.
For the same reason, this particular design cannot properly control the exact angular return stroke since, depending on the actual release torque of the clutch spring towards the belt tightening direction, the spring tang and the first two coils adjacent to this tang are bending and opening unpredictably therefore changing the actual stroke angle between the stops.
The design, described in U.S. Application No 60/335,801 does provide a practical and reliable solution for releasing the tensioner arm away from the belt for reinstallation purposes. However, this design, similarly to the three above-mentioned ones, requires a multitude of expensive, high precision components which also increase the size and the weight of the tensioner, all this highly undesirable in modern motor car engines. This design also has the disadvantage of not controlling the release torque well enough. Although the first coils are prevented in opening excessively, the each and every one of the rest of the coils—when being opened to release the clamping fit on the clutch core—must still take a support on the core therefore increasing the frictional drag which increases the peak values and variations in the release/back torque of the clutch.
While the use of one-way clutches together with the built-in limited thermal expansion stroke features have made it possible to simultaneously use the main spring to automatically take care of the installation stroke and to extend the overall operation range of the tensioners thereby allowing longer belt stretch, this design concept has introduced another so far unsolved technical problem. In order to control the belt tension over a wider angular stroke has made it necessary to use main springs with much lower spring rates than what is case in the conventional short stroke tensioners. The lower spring rates dictate the springs being much longer—i.e. having more coils—and the prewinding angles have become larger. These spring designs have a distinctive drawbacks, the natural frequency of the spring gets lower and springs are easily distorted when wound with large preload angles. This, on the other hand, make the springs much more suspect for excessive vibrations during the service leading to premature fatigue failures of the spring resulting in damaged engines.